As a graduate student, teaching biology labs is a regular part of my week and the lab that I teach most quarters is on phylogenetics and biodiversity. Recently I have been having discussions about phylogenetic terms with my students, fellow TAs, and the staff and professors who are in charge of the class. Terminology often gets confusing in phylogenetics, and words are sometimes used to mean different things by different people. One of the facets that makes these terms extra confusing is that they are not mutually exclusive and depend on the groups being discussed, so for example, a trait can be both a synapomorphy and an autapomorphy (see below) depending on the groups being examined. So, in an attempt to clear things up in my own mind, and so hopefully be able to teach them more effectively, here are some commonly used terms in cladistics with accompanying definitions and explanations.
Apomprphy – A derived character state. This is anything that is an innovation along an evolutionary linage. So anything that is different from the ancestral character state. For example, within the phylum Chordata, the evolution of a vertebral column, which is something lineages that branched off earlier in Chordate evolution do not have and so is new in the Class Vertebrata, would be an apomorphy.
Synapomorphy – A shared, derived character state. This is an apomorphy that two taxa share and that is assumed to have been present in the common ancestor of those two taxa. An example would be feathers in birds. All birds have feathers, and it is assumed that they have feathers because the common ancestor to all birds had feathers and passed that characteristic down through the generations.
Plesiomorphy – An ancestral character state. This is any trait that was inherited from the ancestor of a group. For example, reptiles are exothermic, they do not maintain a constant internal body temperature. They have this characteristic because the ancestor of all reptiles was exothermic. This differs from a synapomorphy because some descendants of the first reptiles are not exothermic (birds are endothermic). In other words, this trait is ancestral, but is shared by some, but not all, of that ancestors; descendants.
Symplesiomorphy – A shared, ancestral character state. This is any trait that was inherited from the ancestor of a group and has been passed on into more than one descendant lineage. To carry on with the example for a plesiomorphy, the fact that crocodiles and turtles are both exothermic, but
Autapomorphy – A derived trait that is unique to a particular taxa. These are not useful in determining how groups are related since only one group will have the particular trait. However, these are extremely useful in identifying taxa. For example, feathers only occur in birds. This makes the character “feathers” and autapomorphy for class Aves. The character “feathers” is also a synapomorphy for taxa within class Aves. Raptors and songbirds both have feathers and they inherited them from a common ancestor.
Open circles = ancestral character state, filled circles = derived character state.
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